Intensifying x-ray film cassettes are widely used in radiography to hold light-sensitive film against a prompt emission phosphor screen, commonly referred to as an "intensifying screen." The cassettes are generally fight tight to permit radiographic procedures to be carried out in normal room lighting. X-ray film cassettes are subject to a number of constraints. X-ray film cassettes must be both relatively light in weight and at the same time physically robust. X-ray film cassettes must be easy to load and unload. X-ray film cassettes must also support the formation of an image of good quality on the exposed film.
Imaging issues are of particular concern in areas like mammography where small differences in x-ray absorption must be visualized in surrounding soft tissue. This has lead to a differentiation of x-ray cassettes on the basis of the number of intensifying screens used in the cassette. In mammography and other specialty fields, x-ray cassettes are typically fitted with a single intensifying screen and the film used has light sensitive layers on one side only, in close proximity to the intensifying screen. In general radiography, x-ray cassettes are typically fitted with two intensifying screens; and an x-ray film with light sensitive layers on both sides is placed between the screens. In the general radiography cassette, both light sensitive layers receive light from both screens. The provides an increase in speed relative to a comparable cassette, but a relative decrease in resolution.
In x-ray film cassettes, the sharpness of the film image is also related to the separation of the light sensitive layers of the film and the luminescent layer of the intensifying screen. Close contact is commonly provided by pressing the film and screen against each other by means of a compressed resilient pad under each screen. In a typical general radiology cassette, internal contact pressures are up to about 0.12 psi. In a typical mammography cassette, internal contact pressures are up to about 0.09 psi. The difference in pressures is related to the different materials used for the tube side panels in the cassettes. The typical general radiography cassette has a tube side panel of 0.063 inch thick aluminum. This material is substantially transparent to x-rays at the wavelengths used in general radiography, short wavelength x-rays generated by an x-ray tube at an applied voltage of 80 to 100 kilovolts. In mammography, the applied x-ray tube voltage is approximately 27 kilovolts and relatively long wavelength x-radiation is produced. The aluminum tube side panel described above is too x-ray absorptive for use in a mammography cassette. Cassettes for mammography and the like commonly utilize polymeric tube side panels. A widely used material is polycarbonate. Carbon fiber-polymer composites are also used.
Mammography cassettes are commonly manufactured by forming or attaching cooperating labyrinthine light lock features at the perimeters of two panels, attaching one sidewall of each panel to a common hinge means, and providing latching means along the panel sidewalls opposite those attached to the hinge means. Typically, such cassettes contain a single prompt emission stimuable phosphor intensifying screens, to produce an acceptable image at relatively low patient exposure levels. The screen is normally affixed to the surface of a compressible pad, which, in turn, is affixed to the interior surface of one of the cassette panels.
U.S. Pat. No. 5,146,484 discloses a cassette having a continuous polymeric hinge affixed to one side. U.S. Pat. No. 4,467,919 and U.S. Pat. No. 4,637,043 disclose mammography cassettes having a film hinge on a thin wall and a light lock on three sides. A compression pad on the cover portion moves a film sheet toward the thin wall during closing of the cassette.
U.S. Pat. No. 4,681,227 discloses a storage phosphor (nonphotographic film) cassette having an embossed resin sheet overlaid on a cushion pad. U.S. Pat. No. 5,025,465 discloses a storage phosphor cassette having a similar "foil" applied on the surface of a pad.
A problem presented by previous cassettes is that, when manufactured, intensifying screens tend to have puckers or dimples or other flatness disturbances. In the manufacture of intensifying screens, several wet layers are coated on the surface of a flexible plastic web. Drying of the coatings results in shrinkage, which causes the coated web to curl significantly. This curliness is overcome by coating and drying layers with similar shrinkage characteristics on the back surface of the web. The resulting web, when cut into discreet sheets, on a macroscopic level, lays relatively flat, however, nonuniformities in the coated layers, and in the drying of the coatings, coupled with the high stress induced by the shrinkage of the coatings cause small localized disturbances in the flatness of the screen surface. These flatness disturbances tend to be very small, but are generally sufficient to degrade image quality of the resulting x-ray mammography film. Intensifying screens adhered to compressible pads tend to retain these flatness disturbances. On the other hand, intensifying screens adhered to hard panels tend to develop flatness disturbances if dust or debris is present between the screen and the panel.
The x-ray film cassette must be capable of withstanding internal pressures present in the closed cassette and the external pressure applied to close the cassette, without film-screen separation, or cassette distortion or break-down. Especially in mammography, intimate contact between the light sensitive layers of the film and the intensifying screen is essential to produce the required sharpness in the image, and separations between the film and the screen as small as 0.0005 inches can seriously degrade the image quality. A number of past cassettes have pre-curved one or both panels, or an internal pressure plate, to ensure good film-screen contact and deter bulging of the closed cassette.
U.S. Pat. Nos. 4,081,686; 4,538,294; and 4,951,306 disclose cassettes having two differently curved panels.
U.S. Pat. No. 3,958,125 discloses a cassette having a convex curved, flexible cover panel and a substantially rigid lower panel.
U.S. Pat. No. 4,032,790 discloses a cassette having one panel curved in two directions and other panel flat. A cushion pad is attached to one panel. U.S. Pat. No. 4,538,294 comments on a shortcoming of the cassette disclosed in U.S. Pat. No. 4,032,790:
"The back plate 54 is usually formed of an aluminum plate. It is difficult, however, to process an aluminum plate into a three-dimensional curved surface having elliptic contour lines. Since the intensifying screens 58 are rigid, they cannot easily be bent in conformity with the three-dimensional curved surfaces. Thus, it is hard to bring the X-ray film 62 uniformly into contact with the intensifying screens 58 on the back plate 54. Also in this cassettte 50, the joining of the front plate 52 and the curved back plate 54 requires great force." (U.S. Pat. No. 4,538,294, col. 2, lines 42-51)
U.S. Pat. No. 4,538,294 provides panels each beating a curved cushion member. Curvatures of the cushion members are at right angles.
A film cassette having a pre-curved polycarbonate bottom panel and a precurved pressure plate is disclosed in U.S. Pat. No. 5,388,140. Precurvature is determined by finite element analysis techniques based on loading forces on the closed cassette. An intensifying screen is adhered to the pressure plate and a compressible pad is adhered to the bottom panel.
U.S. Pat. No. 5,265,148 discloses a mammography cassette having a curved lower panel, an upper panel including a plate bonded to reinforcing webs, light lock structures on three sides and a hinge on the fourth, thin side.
Many current cassettes tend to entrap of air between the film and the screen when the cassette is closed. This causes one or more areas of poor contact between the screen and film. For many cassettes, as time passes, the contact pressure exerted by the compressed foam pad displaces much of the entrapped air, however, it is desirable to minimize the time required for air purging.
U.S. Pat. No. 4,386,431 discloses a magnetic-plate-type cassette having an intensifying screen with a protective layer having a surface roughness of 10 to 60 micrometers. The roughness allows outward air flow from between a sheet of film and the screen. The roughness is provided by embossing or using particles in the range of from 15 to 60 micrometers. Both walls of the cassette and the magnetic plate have a slightly cylindrical pre-bend when the cassette is open. The convex sides of the bends face toward one another. A steel plate opposite the magnetic plate is approximately flat.
U.S. Pat. No. 4,166,958 discloses a cassette including a cushion pad having air channels for air release.
U.S. Pat. No. 4,059,768 discloses the use of solid particulates in the protective layer of an intensifying screen. The protective layer has a static friction coefficient (.mu.) at 20.degree. C. not higher than 0.50 on steel. The protective layer has a micro-unevenness in the range of at least 3 micrometers and preferably from 5 to 10 micrometers. The particles have a preferred average particle size of between about 5 and 25 micrometers. The thickness of the binder in the protective layer is adjusted to provide the desired micro-unevenness. For example, a protective layer with 8 micrometer beads has a thickness of 5 micrometers between beads.
It would thus be desirable to provide an improved film cassette that has a high internal pressure, but does not have the complexity of two precurved panels.
It would thus be desirable to provide an improved film cassette that has a rapid air purge rate.
It would thus be desirable to provide an improved film cassette that has a substantially uniform and relatively small screen-film separation.
It would thus be desirable to provide an improved film cassette that is simple to use.
It would further be desirable to combine all these features.